Most towbars are fitted with shear pins for good reason. Tugs are powerful and aircraft sometimes refuse to move. If you try hard enough you can badly damage the nose gear by pulling too hard. The shear pin is designed to let go, hopefully, before the nose gear comes out by the roots. All well and good if this happens when you are just starting to move. You merely look silly driving away with a short piece of towbar attached. Not so good if you are in transit from the hangar to the gate and it shears as you are crossing runway 25L with the tower asking you to expedite as there is a B747 turning on to final.
When the pin shears, it releases the aircraft. It is very disconcerting for the tug driver to be moving along and realize that the airplane is no longer attached to his tug - especially if it catches up with him. It is even more disconcerting for the mechanic riding the brakes upstairs in the cockpit as he looks out and sees the tug going left while he and his airplane are not. Even more fun occurs if the hydraulic accumulator has gone flat, leaving him with a hand pump to work to build up pressure. This was in pre-APU days, of course. The old piston aircraft weren't blessed with them nor were the first generation jets. And, they didn't even have hand pumps - only the emergency air brakes.
There appears to be a better way now. Way back when, we used a "rocket launcher" to get Electras into Hangar 9 at Idlewild. The tail was slightly taller than the doors so the big scoop of the rocket launcher was backed into the nose wheels to scoop them off the ground. A bar was installed behind the wheels to hold them in place and an electric pump raised the scoop up about five feet. The tail went down and the rocket launcher with its captive Electra was towed into the hangar. It was then lowered as there was plenty of room for the tail inside.
The present day "towbarless" tractors work on the same principle except that they don't have to raise the airplane. I like the concept except it came along a bit late to help me. No humping a heavy bar up onto the nose gear lugs or fighting to make a lock mechanism engage. No straining to lift the eye end of the towbar up while directing a tractor driver to back up to engage the hook. No shear pin letting go at the wrong time. Best of all, no running around looking for the right towbar.
This equipment may not mean the elimination of the towbar but for certain applications, the towbarless tractor fits the bill.
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